Services: Add more ToString() and ToDescription() methods

This is a temporary solution to make these strings re-usable in the
WebFrontend. David Blain will be implementing something much more
sophisticated when he has time.
WebFrontend: Initial work on schedule editor. (Incomplete)

Refs #11938
Eliminate use of the recgroup string from RecordingRule.

I've added some temporary static helpers to RecordingInfo allowing
conversion between ID and string representations of the recording
group to ease the transition.

NOTE: Changing the recording group of a rule via MythWeb is almost
certainly broken after this change and I've no intention of fixing
since the WebFrontend will have a working schedule editor very
soon. If anyone wants to keep MythWeb alive then I suggest porting it
to use the services API for schedule work.